This invention relates to methods of manufacturing composite structures fabricated by joining materials of dissimilar or different kinds or types and to be utilized, for example, as a secondary plate of a railway track of a linear motor car for inducing an electric driving force for moving such motor car.
As is well known, traffic facilities have occupied a significant and important position in society, and among various traffic facilities, transportation by railway plays an important role in rapid mass transportation, apart from transportation by automobiles.
However, as the speed of railway transportation has been increased, vibrations and noises that result have become factors that cannot be neglected by society. Another aspect of railway transportation is the greater and greater costs of increasingly larger scale, excavating and drilling operations for underground railway tunnels. For these reasons, development of a linear motor car capable of travelling along an iron railway track without generating violent vibrations or noises is highly desired and has been partially realized.
In connection with the linear motor car of the character described above, a structure of a secondary plate, which functions to produce eddy currents, in a primary side of the motor car for producing a driving force, must be used on the railway track. Such secondary plate is required to have high electric conductivity and magnetic permeability as well as a great resistance against shearing forces and separating forces which are generated as counter forces against the great driving force. However, in the current technique, a single metal or metallic material having these characteristics has not been developed, and accordingly, a composite structure fabricated by tightly and firmly joining materials of different types and having a high electric conductivity and a high magnetic permeability, respectively, is utilized as the secondary plate.
For example, there are known composite structures fabricated by fixedly joining face to face an aluminum plate having high electric conductivity on a soft steel plate having high magnetic permeability. In an example of such composite structures, the aluminum plate is press-fitted at opposite ends thereof on two end surfaces of the soft steel plate, and fastening screws are passed through the aluminum plate into the soft steel plate to mechanically join the plates as a combined metallic structure. In another example, a composite structure is obtained by joining the aluminum plate to the soft steel plate by an explosion bonding method into an explosion bonded clad plate.
However, as described hereinbefore, it is required for the secondary plate to posses mechanical strength against the strong reacting forces to the driving force, acting frequently and repeatedly, and accordingly, the secondary plate must have a large resistance against shearing forces along the interface between the two elementary plates as well as against vertical separating forces imparted to the two elementary plates of the secondary plate. Particularly, when a composite structure is employed with a railway track of a subway, it is required for the composite structure to have mechanical strength or resistance against additional thermal behavior and separating forces due to temperature variations and dynamic vibrations. However, the composite structures fabricated by pressforming the two plates and then clamping by fastening screws or fabricated by the explosion bonding method cannot maintain sufficient resistance for a long period of time.
Particularly, the composite structure, in which an aluminum plate and a soft steel plate are mechanically joined by pressing them and clamping them by means of bolts, tends to be loosened as time elapses, while the explosion bonding method is extremely expensive, thus being not advantageous.
Moreover, when the composite structures of the characters described above are applied to the railway track of subways, for example, sited near a seaside, the railway tracks are exposed to severe environmental conditions such as corrosion or submergence, and the interface between the soft steel plate and the aluminum plate is corroded. When corrosion occurs, the composite structure cannot perform the function of a secondary plate for a railway track.